The present invention relates to the field of eye examination apparatus, in particular apparatus for measuring the visual function of the eye.
In the field of ophthalmology, the perimetry technique is known for measuring the visual function of the eye. According to this methodology, the patient is shown with light stimuli of various shape and intensity located in selectable positions of the field of view and superimposed on a light background of uniform intensity.
During the examination, the patient looks toward a light fixation target, so as to hold the eye still, and indicates whether he/she sees the light stimuli projected.
In this way, it is possible to determine the minimum threshold of light intensity that the patient's eye is still able to see in various different points of the field of view.
This allows tracing a map of the visual sensitivity of the eye to be used for medical diagnosis.
Numerous examples are known of eye examination apparatus, commonly known with the term “perimeter” and which allow implementation of the perimetry technique for measuring visual function, as described above.
Some of these apparatus comprise a system for projecting a uniform light background on the retina and a system for projecting visible stimuli that can be selectively positioned inside the field of view of the eye.
In some prior art perimeters, these projection systems are operatively associated with a system that allows images of the retina to be acquired.
Apparatus of this type are characterized by high precision in positioning of the light stimuli, given that, in calculating the position of these latter, it is possible to compensate for any movements of the patient's eye.
Moreover, in addition to the result of the perimetry test, these apparatus are capable of providing images of the eye fundus. This is often useful for medical diagnosis.
Examples of perimeters capable of acquiring images of the retina are described in the patent documents U.S. Pat. No. 6,705,726, U.S. Pat. No. 7,690,791 and WO2010113193.
U.S. Pat. No. 7,690,791 describes a combination between a confocal system for acquiring images of the retina and a display that can be positioned in front of the patient's eye. Light stimuli that can be selectively positioned in the field of view are projected on the display.
The document does not provide practical solutions for producing optical integration between the aforesaid display and the confocal acquisition system along a common optical path directed toward the patient's eye.
U.S. Pat. No. 6,705,726 describes the combination between an image acquisition system of the retina and an LCD display capable of showing the patient a uniform light background, a fixation target and light stimuli to test visual function of the eye or perform other tests.
The image acquisition system (which in this case is not of confocal type) illuminates the patient's retina with infrared light and records video of the same retina.
It has been seen that this acquisition system often provides low contrast infrared images, of little use for medical diagnosis and difficult to use to detect eye movements in real time.
The use of a LCD display to project the light stimuli has some disadvantages. The most important of these are:                low precision of the intensity of the light stimuli: light stimuli projected from different regions of the surface of the display, which can have different characteristics from one another;        low interval of variation (dynamic range) of the intensity of the light stimuli: in general it is possible to obtain a variation of around 30 dB between a minimum and maximum intensity of the light stimuli projected (with other solutions that do not use liquid crystals it is possible to reach intervals of variation of around 50 dB);        discrete form of the light stimuli: the light stimuli projected are typically composed of a few pixels;        variations in the light intensity on the surface of the stimulus (light regions at the surface of the pixel and dark regions at the boundaries between adjacent pixels);        variation of intensity of the light stimuli with the temperature of the LCD display.        
Patent application WO2010/113193 describes a scanning perimeter comprising an acquisition system of images of the retina of confocal line scanning type, which uses infrared light to illuminate the retina. This acquisition system is operatively associated with a projection assembly, coupled with the optical path of the machine by means of a cold mirror. The projection assembly comprises optical elements that separately produce a fixation target, a uniform light background and light stimuli to measure the sensitivity of the retina.
The apparatus described in this patent document has considerable disadvantages in terms of structural complexity and costs for its production on an industrial scale.
As well as the line scanning acquisition system, this apparatus comprises a projection assembly that, among other things, provides for the use of a plurality of emitters, two beam splitters, a cold mirror, at least two lenses and a two-way electromechanical system for movement of a mirror that modifies the position of the visual stimulus projected on the retina.
To project a uniform light background, the aforesaid scanning perimeter uses a light surface optically conjugated with the retina, which generates light with uniform intensity.
This solution is generally costly, given that the uniformity of the light background generated depends substantially on the quality of the components used.
Moreover, the light background projected can have non-uniform regions due to dirt that can deposit on the aforesaid projection surface, which would be visible to the patient in the form of dark marks with well-defined outlines.